Get no results when using search issues API with keyword which length less than 3

I use issues search API to list issues in a project.
Search keyword’s length is greater than 2, it works normaly.
But when the seach keyword’s length is less than 3, I can’t get any result.

GET /issues?search=fo

Is it a bug in GitLab API?

GitLab version: GitLab EE 11.6.11 or 13.4.1